A rotary electric machine is a general term for an electric motor, an electric generator, and an electric motor and generator. When an interior magnet rotary electric machine of the present invention is utilized as an electric motor, such an electric motor is called an IP (Interior Permanent Magnet) motor. On the other hand, an electric motor including a rotor in which a permanent magnet is attached to a surface of a rotor core is called an SPM (Surface Permanent Magnet) motor. Furthermore, the IPM motor and the SPM motor are collectively called PM motors.
Patent Literature (PTL) 1 discloses a rotary electric machine that includes holding ring disposed on the surface side of permanent magnets, although the disclosed rotary electric machine belongs to the SPM motor instead of the IPM motor.
When a rotor is rotated, a centrifugal force acts on the permanent magnets attached to the surface of the rotor core. Because the centrifugal force is proportional to the square of an angular speed, a rotary electric machine with a rotor rotating at a high speed has a problem that the permanent magnets may be detached and scattered to the surrounding. In PTL 1, the permanent magnets are firmly fixed to the rotor core by arranging the holding ring on the surface side of the permanent magnet.
Meanwhile, there is a design method of reducing the size of a rotor core in order to reduce a centrifugal force that acts on permanent magnets. For the reasons described below, such a design method is applied in a larger number of cases to an SPM structure in which the permanent magnets are attached to the surface of the rotor core than to an IPM structure in which the permanent magnets are embedded in the rotor core.                (1) Because the size of the rotor core is small, the IPM structure in which the permanent magnets are embedded in the rotor core has a difficulty in arranging each pair of adjacent magnets in such a state that a spacing between the magnets spreads gradually toward the outer peripheral side as in a 4-pole rotary electric machine of related art.        (2) In the IPM structure, a width of a magnetic flux short-circuit path, which is defined by a portion between an outer periphery of the rotor core and the magnet, in a radial direction of the rotor core needs to be increased from the viewpoint of giving the rotor core with a sufficient level of strength endurable against the applied centrifugal force. However, an increase in the width of the magnetic flux short-circuit path increases an amount of short-circuited magnetic flux passing inside the rotor core, and reduces an amount of magnetic flux coming into a gap between a stator and a rotor. Hence torque is reduced.        (3) A larger output power can be obtained as the number of poles increases. However, as the number of poles increases, a higher driving frequency is required and a larger induced voltages is generated in windings.        
On the other hand, in the SPM structure, because the permanent magnets are attached to the surface of the rotor core, an eddy current loss is larger than in the IPM structure in which the permanent magnets are embedded in the rotor core. Moreover, reluctance torque can be utilized in the IPM structure, while reluctance torque cannot be utilized in the SPM structure.